Typescript interface with dynamic keys extending Object

Question

In Typescript I am not able to make my interface extend Object when indexer is used (key as string).

If i do not extend Object then it works fine , but intellisense does not give suggestions for Object.hasOwnProperty() method.

interface MyObject extends Object {
 [key: string] : string;
}

Above code, I get compile time error as: "Property 'hasOwnProperty' of type '(v: string) => boolean' is not assignable to string index type 'string'."

Later in code i would like to use variable of type MyObject to check if it contains a particular key using hasOwnProperty method of Object.

Solution

You don't need to extend Object to have the hasOwnProperty method. Since all objects inherit Object this method will exist on any instance of the interface.

interface MyObject {
    [key: string]: string;
}

var v: MyObject = {
    "foo" : "1"
}
v.hasOwnProperty("foo");

The index signature generally means that all members of the interface will be compatible with the return type of the index. You can get around this using union types though, but you still can't directly create such an object without Object.assign:

type MyObject  = Object & { // Object is useless but we can specify it
    [key: string]: string;
} & { // We can specify other incompatible properties
    required: boolean
}

// We can create an instance with `Object.assign`
var v: MyObject = Object.assign({
    "foo" : "1"
}, {
    required: true
});
v.hasOwnProperty("foo");
console.log(v.required);
console.log(v['bar']); 

Comments

[Indra Chatterjee]

Thank you, this gives much clarity needed. Just that when i do "v.", in code i do not get hasOwnProperty in suggestion list. Is there a way around to that ?

[Danielo515]

This does not work. Which version of TS you tested this with?